Foamed, rigid or structural thermoplastic moldings offer basically the same wide range of attractive properties as do unfoamed rigid thermoplastic moldings plus lower density, high strength to weight ratios and freedom from sinks, warping and molded-in stresses while overall shrinkage remains about the same as for solid moldings. A sink and stress-free part is a major advantage of a structural thermoplastic foam molding over a structural thermoplastic solid molding.
The foamed structural thermoplastic moldings may be made by high pressure or low pressure processes.
High pressure processes (cavity pressures of about 1,000 to 5,000 p.s.i.) can make structural foam parts with fair to excellent surfaces and with thinner walls than with a low pressure molding process. Also, high pressure molding is said to give 25 to 50% shorter cycles, better reproduction of mold detail and to be readily converted to conventional injection molding. However, cavity pressure is about the same as that for conventionally injection molded solid thermoplastics so that the maximum part size is ordinarily limited to about 25 pounds which is approximately the same as that obtained with solid parts. Also, the high pressure process requires that the mold be completely packed with resin and then the mold be expanded to allow the molten resin to foam. Unless the part to be made is essentially flat, this requires the mold to be expanded in several directions which may be impractical. An alternative to this is to withdraw part of the melt, but this is little practiced. Furthermore, a blemished area is left on the part where a new mold area is exposed as the mold expands. These problems restrict the use of high pressure processes since the advantages of foaming rarely outweigh the drawbacks where solid part high pressure molding is an alternative.
Many manufacturers use the low pressure processes to form structural thermoplastic foam moldings or parts, because this method permits making giant parts (up to 100+ pounds) and so enables the consolidation of many small parts into one unit. In this process, the mold is undershot (incompletely filled) and the charge fills the mold by free expansion of gas filled charge or melt on heating. Cavity pressures in the low pressure process may be very high initially at the point of injection but finally average about 300 p.s.i. Thus, the press and mold costs are less for the low pressure process which additionally can make large parts and profitable short runs. Nevertheless, only about 10 to 20 inches of flow from injection points is normally possible when using the low pressure process, and, even for this, very fast injection is required. Therefore, large part molding is best performed with specialized injection molding machines distinguished by a large hot manifold with multiple injection nozzles. The multiple injection points required and the tendency towards large inclusions of air demand superior molding skills.
The most serious limitations of thermoplastic structural foam parts made by the low pressure process are the rough (1000.mu. inch variations), wrinkled, mottled surfaces and inability to effectively foam sections less than 0.2 inch. A wood-like surface is especially easy to obtain, but high gloss (like lacquered metal) surfaces are prohibitively expensive. The surface can be improved or masked by sanding, filling, priming and painting, but this is labor intensive and may represent 1/3 to 1/2 of the total part cost if high gloss is desired (for office machine covers, automobile body parts and so forth).
Helpful but limited approaches to achieving smoother surfaces of varying degrees which may not be suitable for all part shapes include (1) delayed cooling by covering the mold cavity with a heat insulating film, (2) painting the mold cavity and allowing the paint film to transfer to the molding, (3) injecting into a plastic shell preinserted into the mold, (4) injecting gas-free melt ahead of the foamable melt so as to encapsulate the foam formed with a solid skin, (5) pressurizing the mold enough to reduce evolution of gas at the front of the injected melt and (6) injecting into a hot mold and then cooling the mold.
It, therefore, is an objective of the present invention to attempt to overcome the difficulties alluded to hereinabove and to provide a method for in-mold coating a thermoplastic structural foam molded part of a polycarbonate resin or an ABS polymer resin so that the part exhibits an improved surface.
These and other objects and advantages of the present invention will become more apparent to those skilled in the art from the following detailed description and working examples.